Data transmission by means of visible light (“visible-light communications”, VLC) is known, which can be used, for example, to supplement conventional radio engineering. In this case, data can be transmitted via light-emitting diodes (LEDs), for example. In this case, a data stream to be transmitted is transmitted in the form of modulations imperceptible to humans.
Also known is a more recent encoding method for visible light based on color coding with elementary colors. Experts also refer to this kind of method using the term CSK (“color shift keying”). Other older names for this encoding method are CCM (“color code modulation”) or CMC (“color multiplex coding”).
Roughly speaking, the functional principle of VLC using this color coding method consists in using mixed lighting comprising a plurality of elementary colors for the additional transfer of data, wherein the respective elementary colors are modulated so quickly that in total the human eye recognizes a continuous mixed color. Usually, the three elementary colors red, green and blue are used for this, which can be emitted by sophisticated technology by means of corresponding light-emitting diodes.
A detailed description of CSK can be found in the proposed modification to the standard IEEE P802.15.7, Yokoi et al.: “Modified Text clause 6.9.2.2”, 17 Jan. 2010, Document Identification “15-10-0036-00-0007”. One of the proposed applications for CSK applications is VLC, that is free-space communication with light.
Said proposed modification describes automatic compensation on the receiver side for the changes to the optical power of the elementary color light-emitting diodes provided on the transmitter side. Changes of this kind occur, for example, due to ageing phenomena in individual light-emitting diodes, which, with an increasing service life with the same electrical power supply, are only able to emit a lower optical power. A change of this kind is accompanied by a proportional change in the quantum efficiency of the transmitters. According to this proposed modification, as the compensation is only on the receiver side, obviously there is no compensation of the mixed color, that is of the spectrum of the emitted radiation.
Therefore, there is a requirement to compensate changes to the optical power of the elementary color light-emitting diodes provided on the transmitter side not on the receiver side, for example by adjusted sensitivity of radiation receivers, but on the transmitter side, for example by a higher supply of electrical energy with reduced quantum efficiency.